Rhodamine dyes fluoresce and have been used extensively in research, both as free dye and as conjugates to larger molecules, e.g. proteins and antibodies (Lee S, McAuliffe D J, Kodama T, Doukas A G, In vivo transdermal delivery using a shock tube, Shock Waves (2000) 10:307-307; Janson L W, Ragsdale K, Luby-Phelps K, Mechanism and size cutoff for steric exclusion from actin-rich cytoplasmic domains., Biophys J (1996) 71:1228-1234; Pu R, Robinson K R, Cytoplasmic calcium gradients and calmodulin in the early development of the fucoid alga Pelvetia compressa., J Cell Sci (1998) 111 (Pt 21):3197-3207; Nishiya T, Kajita E, Horinouchi T, Nishinioto A, Miwa S, Distinct roles of TIR and non-TIR regions in the subcellular localization and signaling properties of MyD88, FEBS Lett (2007) 581:3223-3229; Tanner G A, Sandoval R M, Dunn K W, Two-photon in vivo microscopy of sulfonefluorescein secretion in normal and cystic rat kidneys, Am J Physiol Renal Physiol (2004) 286:F152-F160).
Structurally, rhodamine is a family of related polycyclic flurone dyes with a xanthene core.

The general structure of rhodamine is as follows:

The amines of rhodamine can be primary amines, secondary amines or tertiary amines.
One of the commonly used fluorescent rhodamine dye is sulforhodamine 101 which contains a julolidine structure element:

Sulforhodamine 101 contains bi-functional sulfonyl groups as shown below:

Sulfonylrhodamine 101 has been used in neurophysiological experiments which comprise calcium imaging methods as well as a counterstaining of astrocytes (Nimmerjahn, A., Kirchhoff, F., Kerr, J. N., Helmchen, F., Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo, Nature Methods (2004) 1:31-7).
A sulfonyl chloride derivative of sulforhodamine 101 is sold by Sigma Aldrich, Inc. (St. Louis, Mo.) under the trademark. Texas Red®. It is used for conjugation with a number of functional groups, especially with primary amities. Texas Red® fluoresces at about 615 nm with a peak absorption at 589 nm. Texas Red® is typically available as a mixture of two monosulfonyl chlorides with the SO3 and SO2Cl groups exchangeable as shown below:

Other rhodamine derivatives have also been disclosed, such as in PCT Int. Appl. (2009), WO 2009108905 A2 2009090; U.S. Pat. Appl. Publ. No. 2004054162 A1 20040318; U.S. Pat. No. 5,728,529; U.S. Pat. No. 5,686,261; PCT Int. Appl. (1997), WO 9700967 A1 19970109; U.K. Pat. Appl. (1995), GB 2283744 A 19950517; and by Kim etl al. (Kim, T. G.; Castro, J. C.; Loudet, A.; Jiao, J. G.-S.; Hochstrasser, R. M.; Burgess, K.; Topp, M. R., Journal of Physical Chemistry A (2006), 110(1), 20-27).
Although several publications show the possibility of using bi-functional rhodamine dyes for conjugation, e.g. by selective reaction of one of the sulfonyl chloride groups in Texas Red® (Titus J A, Haugland R, Sharrow S O, Segal D M, Texas Red, a hydrophilic, red-emitting fluorophore for use with fluorescein in dual parameter flow microfluorometric and fluorescence microscopic studies, J. Immunol. Methods (1982) 50 (2): 193-204), the possibility of dual reactivity gives difficulties in establishing a reliable process yielding a single isomer product only, which is shown in FIG. 1 in which a bi-functional rhodamine dye with two sulfonyl groups reacting with a primary amine to form two conjugation isomeric products. Two isomeric structures instead of only one in a process gives two major disadvantages: 1) the ratio of isomers changes from batch to batch with product impacting effects, and 2) the regulatory requirements (toxicity data, stability, characterization, etc) will be doubled to cover two substances instead of one.
The present invention circumvents these difficulties by using novel mono-functional derivatives of rhodamine dye with only one single functional group on the rhodomine molecule for conjugation so that their conjugation products are single isomeric conjugation products.
These and other aspects and attributes of the present invention will be discussed with reference to the following drawings and accompanying specification.